Lacunar Dilatations of Intrafusal and Extrafusal Terminal Cisternae, Annulate Lamellae, Confronting Cisternae and Tubulofilamentous Inclusions within the Spectrum of Muscle and Nerve Fiber Changes in Myotonic Dystrophy

In 3 out of 5 muscle spindles available in skeletal muscle biopsy specimens from 30 patients with myotonic dystrophy (MD) unusually large lacunar dilatations of terminal cisternae were observed that had thus far only been reported in extrafusal muscle fibers. Cytoplasmic annulate lamellae, confronting cisternae and regularly proliferated terminal cisternae, as well as intranuclear tubulovesicular inclusions were found in extrafusal muscle fibers that in combination with concentric membranous bodies seen in perineurial cells and Schwann cells generally emphasize an involvement of the endoplasmic reticulum in the pathogenesis of MD. In addition, a nuclear inclusion body was observed composed of tubulofilamentous structures with close similarity to those thought to be rather specific for inclusion body myositis. Vesicles filled with amorphous material originating from outer spindle capsule cells were suggested to indicate matrical lipidic debris leading to “ghost bodies” and calcifying globules. Light microscopical evaluation of 8 sural nerve specimens revealed a neuropathy in only 2 patients that was predominantly axonal in type and of slight to moderate severity with a secondary demyelinating component in 1 patient. These findings add to the large spectrum of muscle and nerve fiber changes in MD underlining the phenotypic multiplicity of a well defined genetic defect.